Rending end brittleness remove armor mods from an attack. The benefit of brittleness and rending is dependent on the armor mod of the attack.
So if an attack had 50% armor mod normaly (remove 50% of base damage) and the enemy have total 40% brittleness (max brittleness is 40%) that means that we remove 40% of the armor mod applied. That results in an end armor mod of 70%. (40% of 50% is 20%, 50% +20% = 70%.
If we instead hava a base armor mod of 80% and 40% brittleness we get a new armor mod of 88%. (40% of 20% is 8%, 80%+8% = 88%)
If we had an original armor mod of 0% (you make 0 damage) and brittleness of 40% you would get a new armor mod of 40% (40% out of 100% is 40%, 0% + 40% = 40%)
In this way different weapons (or even attacks on the same weapon) will benefit differently depending on the original armor mod.
Also Rending and Brittleness stacks ontop of eachother, even above brittleness limit of 40%. So if you have 40% brittleness on an enemy and you havea buff that gives 20% rending, the final overall rending/brittleness value is 60%.
I agree that we don’t explain this properly in game. New text for “+Rending” is on its way.
Hope that helps explaining things.
/Guen